(i) Field of the Invention
This invention relates to a device and method for the production, by electrical means, of a sound source suitable for marine applications.
(ii) Description of the Prior Art
Underwater electrical acoustic sources have been used in the past with limited success. Present technology of underwater spark sources requires extremely high voltages in order to electrically break down water. Use of such high voltages is not desirable on board ship. Moreover, conventional spark sources are highly inefficient in converting electrical energy into sound energy.
In addition to conventional electrostrictive and magnetostrictive acoustic sources, spark gaps, thumpers (boomers), and explosives have been used at energies typically in the range of 10-20 KJ. The spark gap sources require voltages of 15-30 KV. Such voltage is required because of the high breakdown voltage of sea water (100-200 KV/cm). Thumper sources were developed as a source of low frequency acoustic energy requiring much lower operating voltages (typically 4 KV). These devices, with efficiencies of 10%, gained preference over spark gap sources primarily on the grounds of safety. Explosives are high energy (megajoule), one-shot sources, and their main drawback is lack of repeatability.
Acoustic spectra of typical spark sources indicate that the acoustic energy is radiated from low frequencies of 100 Hz to a peak at less than 10 KHz. The main parameter affecting the spectra is the rate at which the energy is supplied to the spark gap. The primary conclusion is the spark gap sources closely resemble small (milligram of TNT) explosive sources. Spark sources have not, however, received much attention because of the potential hazard of high voltages.
The patent literature is replete with such devices. For example, Canadian Pat. No. 699,875, patented Dec. 15, 1964, by N. D. Smith, Jr. is directed to a seismic surveying system for water covered areas. The sound source is an extended linear sound source to generate seismic waves in the body of water. Such source is provided with a firing apparatus for igniting or detonating an explosive mixture of gas contained within an elongated flexible member.
Canadian Pat. No. 804,172, patented Jan. 14, 1969, by P. H. Miller, Jr. is directed to an analogue device for geophysical prospecting. The signal generator includes a hollow cylindrical body containing a gas, and means to produce acoustic discontinuities in the gas. Such means is therefore a transducer to generate acoustical waves in the gas.
Canadian Pat. No. 808,298, patented Mar. 11, 1969, by W. A. Kearsley et al is directed to an underground sound-producing system. Such system is of the unmerged sparker type in which a high voltage electrode is spaced from a second electrode by a distance slightly greater than the radius of the plasma bubble produced by the energization of the first electrode.
Canadian Pat. No. 836,712, patented Mar. 10, 1970 by W. H. Luehrmann et al is directed to an apparatus for generating an underwater acoustical impulse. The apparatus includes spacedapart electrodes and contains conductive particles or metallic wires in the region between the electrodes, so that, when stored electrical energy is passed through the region, the metal will vaporize and provide an increased concentration of ions in the plasma region.
Canadian Pat. No. 1,112,349, patented Nov. 10, 1981, by R. A. Kirby is directed to an open ended seismic source. The source is an air gun provided with means for abruptly increasing the pressure of gas in a chamber so that some gas escapes from the chamber to create the pulse. The air gun is also provided with a recoil reducing structure.
Canadian Pat. No. 1,129,980, patented Aug. 17, 1982 by R. C. Johnson et al is directed to an air-release control in a seismic energy source air gun. The air gun has two compressed air chambers and shuttle means for releasing compressed air between the chambers and out into the medium.
Krott et al, U.S. Pat. No. 3,245,032, patented Apr. 5, 1966 provides a structure to give an improved multi-shot acoustic spark source, using a plurality of electrodes, in the absence of a supply a gas.
Kearsley et al, U.S. Pat. No. 3,268,226, patented Nov. 15, 1966 provides an underwater sound source of the unmerged sparker type of a specially recited construction having two electrodes.
Huckabay et al, U.S. Pat. No. 3,304,533, patented Feb. 14, 1967 provides an acoustic source for marine seismic surveying in which two electrodes are placed in a body of water and a series of potentials is imposed across the electrodes to build up the pressure wave.
Berglund et al, U.S. Pat. No. 3,368,643, patented Feb. 13, 1968 provides an electric arc seismic source involving the use of a stream of water and a metallized path between two electrodes.
Luehrmann et al, U.S. Pat. No. 3,369,218, patented Feb. 13, 1968 provides a metallized plasma path source involving the use of a stream of a conductive liquid.
Wright, Jr., U.S. Pat. No. 3,458,858, patented July 29, 1969 provides an acoustic generator of the spark discharge type in which a gas bubble is disposed between two electrodes so that the electric discharge takes place across the bubble. The patentee creates a discharge in a gaseous environment. In the patented device, an attempt is made to place a bubble strategically between the two electrodes at the exact time of electrical switching.
Burrage, U.S. Pat. No. 3,613,823, patented Oct. 19, 1971 provides a double bubble spark array in which the usual ground electrode is replaced by a plurality of parallel electrodes.
Kirby, U.S. Pat. No. 4,193,472, patented Mar. 18, 1980 provides an open ended seismic source involving the use of an explosive chamber fed with an explosive gas and ignited by a capacitance discharge ignition system.